This invention relates to hot melt compositions and particularly to foamable elastomeric hot melt compositions useful in the sealing of containers such as bottles, jars, cans, and the like.
Hot melt compositions, that is compositions which become fluid when heated above ambient temperature and return to a non-fluid or less fluid state when cooled, have found wide use as adhesives and sealants in a number of applications. Often it is desirable that the hot melt composition have a certain degree of resiliency after application, and to this end, methods have been devised to impart a cellular structure to the applied composition.
Traditionally, a surfactant (foaming agent) is added to a thermoplastic polymer to reduce the size of the foamed cells (voids) and cause the voids to be of a more uniform size. For example, in U.S. Pat. Nos. 4,156,754 and 4,259,402, to Cobbs et al, surfactants are added to molten thermoplastic material in order to extend the life or stability of the hot melt adhesive foam.
The use of hot melt compositions having a cellular structure as sealants in closures for containers is described, for example, in U.S. Pat. No. 4,295,573 to Terry et al. In the procedure disclosed in this patent, a heated, pressurized mixture of thermoplastic hot melt composition and a gas such as nitrogen is formed and dispensed to the closure at atmospheric pressure while in a cellular or foamed state. Upon cooling, the cellular state of the applied composition is retained, providing a closure having a resilient sealant material applied thereto.
It is a principal object of this invention to provide a hot melt composition from which a soft, elastomeric, foamed structure having very fine, uniformly sized voids is obtained.